1. Field of the Invention
The invention relates to fingerprint image input devices and living body identification methods using fingerprint images. More specifically, the invention relates to a fingerprint image input device and a living body identification method using fingerprint images that determines whether the finger under scanning is a live or dead one based on the changes in the finger color that changes with the force applied to the finger.
2. Description of the Related Art
Conventionally, the fingerprint image input device has been used for fingerprint identification and user authentication for computer security and the like. For fingerprint identification, however, the fingerprint image input device is used to identify dead bodies as well as living bodies. Because some fingerprint devices do not distinguish between living bodies and dead ones, they allow an unauthorized access to computers, for example, if someone uses counterfeit goods (replicas) of another person's finger for identification. Thus if the fingerprint image input device can distinguish between living fingers and dead ones, it can prevent such unauthorized access to information systems, thereby ensuring ID security. Thus there has been a great need to develop such a living body identification method.
Japanese Patent No. 2708051 (Japanese Patent Laid-Open Publication No. Hei. 3-087981) discloses a fingerprint image input device capable of distinguishing between living and dead bodies. FIG. 1 shows the major constituent elements of the device disclosed in this Japanese Patent No. 2708051. This fingerprint image input device includes a light source 101, a transparent unit 102, a mirror 102a, a lens 103, an aperture 103a, a color CCD 104, an acquired fingerprint image processor 105, an image acquiring circuit 105a, a fingerprint identification circuit 105b, a living body identification unit 106, a color drift correction circuit 106a, a color identification circuit 106b, and an R,G,B separation circuit 107.
Now described is the operation of this prior art fingerprint image input device. The light emitted from the light source 101 passes the transparent unit 102 and irradiates the finger. Part of the light scattered by the finger proceeds in the direction shown by the arrow in FIG. 1, reflects on the mirror 102a and reaches the aperture 103a. The light that has passed the aperture 103a is focused onto the color CCD 104 through the lens 103. The output signal from the color CCD 104 is separated into R,G,B three-color image information by the RGB separation circuit 107 and enters the living body identification unit 106. The living body identification unit 106 detects the changes in finger color resulting from an increase in the finger pressure and determines whether the finger is a living or dead one.
Specifically, this technique utilizes the light reflectivity that changes between the moment the finger has just touched the detector surface and the moment the finger is pressed against the detector surface. In fact, the light reflectivity of a human finger makes a considerable difference in the 450–600 nm wavelength range, depending on whether the finger is depressed or not. At the moment a finger has just touched the detector surface, the image of the fingerprint takes on a tinge of red, while it takes on a tinge of white when the finger is strongly pressed against the detector surface. The color drift correction circuit 106a and the color identification circuit 106b determine whether the finger is a live one or not based on the color difference in the fingerprint image resulting from different finger pressures. If the finger is determined to be a live one, the fingerprint identification circuit 105b in the acquired fingerprint image processor 105 compares its fingerprint image with that registered therein beforehand.
Another living body identification device is disclosed in Japanese Patent No. 2774313 (Japanese Patent Laid-Open Publication No. Hei. 2-307176). This living body identification device utilizes the changes in the color of the finger skin when the finger is pressed against a transparent unit. It determines whether the finger is a live or dead one based on the change in the reflectivity of light returned from the finger when irradiating light onto the finger through a transparent unit from a light source. Such a living body identification device is made thin by disposing the light source, photodetector, focusing device and wavelength selection device under the bottom face of the transparent unit opposite to its top face on which the finger is placed and disposing the light guide unit and light polarization unit further thereunder.
Another fingerprint image input device is disclosed in Japanese Patent Laid-Open Publication No. 2000-20684. It irradiates probe light and reference light from a live-scan light source onto the finger in contact with the detector surface. The detection unit outputs an electric signal corresponding to the intensity of the probe light that has passed inside the finger and another electric signal corresponding to the intensity of the reference light that has passed inside the finger. Then the living body identification unit determines whether the finger placed on the detector surface is a live or dead one based on the intensities of the probe and reference light represented by both electric signals provided by the through-light detection unit.
Those prior art fingerprint image input devices have the following disadvantages.
First, it is difficult to identify living bodies with high reliability. The prior devices determine whether the finger is a live or dead one based on the colors of the fingerprint images corresponding to strong and weak finger pressures. For fingerprint identification, however, some criteria become necessary. Although there are no specific criteria described in the aforementioned patent publication, in general, a finger is determined to be alive when the finger shows a color change larger than a predetermined threshold.
However, when the ambient temperature is low, the finger takes on a tinge of white before depression and thus the finger color does not change much. Also when the finger is lightly pressed against the detector, the change in color is small. In those cases, there is a concern that the obtained color change is smaller than the predetermined threshold and the device makes the wrong decision that the live finger is a dead one. In addition, the identification reliability is much dependent on how a fingerprint image is acquired during strong finger pressure.
In Japanese Patent No. 2708051 (Japanese Patent Laid-Open Publication No. Hei. 3-087981), there is a description “fingerprint image detected with stability by an image detection unit”. However, no specific means for determining whether the image is stabilized or not is disclosed. Depending on the conditions of the finger placed on the detector surface, even a live finger may not bring about a color change large enough for identification. The prior art devices have such a disadvantage of poor capability of carrying out reliable living body identification.
Second, the prior art devices have another disadvantage of being difficult to downsize because they need focusing optic systems. Thus it is not easy to install them in mobile gears such as cell phones and personal digital assistants. The focusing optic system needs a lens, a transparent body, a mirror and other components for guiding light and producing images of little distortion. Thus such a focusing optical system becomes as thick as about the width of a finger. In the lateral direction, the lens must be away from the CCD at least 10 mm for focusing. As a result, it is impossible to install a prior art device in a small space, for example, 1 mm thick and 20 mm wide.